North Bullion occurs in a triangular shaped horst in the footwall of the major north-south striking, steeply east-dipping, Bullion Fault Corridor. The western edge of the horst is bounded by a northeast-striking, northwest-dipping fault. The deposit is blind at surface with the gold system capped by gently east-dipping, dacite sills. In general gold is hosted in two zones: 1) A gently to moderately dipping upper zone of strongly sheared siliciclastic/carbonate rocks (a mixed composite of Mississippian Webb and Tripon Pass Formations) and; 2) A flat lying, lower zone of dissolution collapse breccia developed above and within calcarenite of the Devonian Devils Gate Limestone. This lower zone at North Bullion is the same favorable breccia zone that hosts GSV’s Pinion Deposit 10 km to the south. Between strands of the BFC, breccia with both collapse and tectonic features propagated upwards through the Mississippian section incorporating Webb Formation silty mudstone, Tripon Pass Formation micrite and Chainman Formation sandstone.
Gold zones range from 105 to 400 meters in depth and steepen from flat (10 degrees) to moderate (45 degrees) dips to the east, as they approach the western strand of the Bullion Fault Corridor. Gold is associated with sooty sulfides, silica, carbon, clay, barite, realgar and orpiment in addition to elevated As, Hg, Sb and Tl. High-grade (>6 gpt Au) gold has been intercepted in both the upper and lower gold zones.
Mineralization appears to correlate with a west northwest-striking gravity gradient and mineralization remains open along strike to the west northwest. This flat-lying collapse breccia is the ideal host setting for the classic Carlin-type, sedimentary rock hosted gold systems.
Hydrothermal alteration is characterized by a progression from distal breccia with calcareous clasts and calcite cement to intermediate breccia with dolomitized clasts and dolomite cement to proximal breccia with silica/sulfide cement and replacement of clasts. Dolomitization of the Devils Gate Limestone thickens and strengthens towards the Bullion Fault Corridor. Veinlets show a general zonation around the deposit from outer ferroan dolomite calcite shells to barite +/- quartz immediately above, within and below the gold zones.
Intrusive relationships and tilting of units indicate the deposit formed during an Eocene event with synchronous intrusion, hydrothermal activity and extensional movement on graben-bounding faults. Dacite sills, dated at 38.2-38.8 Ma, intruded steeply dipping faults within the Bullion Fault Corridor and low angle, bedding parallel faults, capping the gold system. The margins of dacite dikes and sills are commonly sheared and some dacite occurs as clasts within mineralized dissolution collapse breccia, indicating continued movement along faults and hydrothermal activity after emplacement of the dacite. In fault steps within the Bullion Fault Corridor, the Eocene Elko Formation has the same moderate eastward dip as the underlying Paleozoic rocks. All of this evidence supports the formation of North Bullion during a very dynamic, focused Eocene event with synchronous extension, intrusion and Carlin-style mineralization.